In recent years, there are high expectations for robots that are operated within areas close to people, such as robots for medical applications, domestic robots, and work support robots for use in factories. Unlike industrial robots, it is important for these robots to ensure safety when the robots are in contact with a person. In order to reduce impact upon contact, for the necessity to reduce a force to act on the contact point, the torque in the joint needs to be controlled to provide a flexible joint when viewed from the robot arm side. However, in the force (torque) control using an actuator for driving the joint, it is not possible to increase the response frequency infinitely, and in the case where force in a high-frequency range is exerted, such as in the case where a robot arm collides with a person, it is not possible to deal with the force. Normally, a combination of a motor and a speed reducer is used for joint driving, and when viewed from the robot arm side, the inertia is represented by a value obtained by multiplying the inertia inherent to the motor by the square of the reduction ratio. For this reason, in a state where force control is not effective, extremely large force acts on the contact point, with the result that it is not possible to sufficiently ensure safety by using only the force control.
In view of these issues, a system has been proposed in which an actuator and a load are connected to each other through an elastic member referred to as “Series elastic actuators (SEA)” (for example, see Patent Document 1). The SEA makes it possible to suppress even force in a high-frequency range that cannot be controlled by actuators, by utilizing flexibility of the elastic member, and also provides a flexible actuator that can achieve a flexible joint at all times when viewed from the arm side, so that it becomes possible to ensure higher safety. On the other hand, since the SEA is connected to a load through an elastic member, its controllable frequency bands are lowered in comparison with a conventional device. In order to compensate for these disadvantages, there has been proposed another system, such as a system referred to as “Distributed macro-mini actuation (DM2)” that is additionally provided with an actuator for high frequencies (for example, see Non-Patent Document 1), or a system referred to as “Variable Stiffness Transmission (VST)” in which the rigidity of the elastic member is variably controlled (for example, see Non-Patent Document 2).    Patent Document 1: U.S. Pat. No. 5,650,704    Non-Patent Document 1: IEEE Robotics & Automation Magazine, Volume 11, Issue 2, pages 12 to 21    Non-Patent Document 2: IEEE Robotics & Automation Magazine, Volume 11, Issue 2, pages 22 to 33